1. Field of the Invention
The present invention relates generally to the production of an x-ray beam by electron impact on a metal target. More particularly, the present invention relates to a transmission type x-ray source that is mobile, miniature, with a configuration allowing placement of a sample close to the point where X rays are generated, with a configuration allowing close placement of a detector in XRF application, and with an electron optical element configuration that allows the generation of a small diameter spot as the source of X rays.
2. Related Art
In an X-ray tube, electrons emitted from a cathode source are attracted to an anode by the high bias voltage applied between these two electrodes. The intervening space must be evacuated to avoid electron slowing and scattering, but primarily to prevent ionization of containment gas and acceleration of the resulting ions to the cathode where they erode the filament and limit tube life. Characteristic and Bremsstrahlung X rays are generated by electron impact on the anode target material. Every material is relatively transparent to its own characteristic radiation, so if the target is thin, there may be strong emission from the surface of the target that is opposite the impacted surface. This arrangement is termed a transmission type X-ray tube. By comparison, a side-window tube has a thick anode in the vacuum space; and its X-ray emission passes from the tube via an X-ray transparent window placed in the side of the vacuum chamber. Each type has its advantages and disadvantages, depending upon the intended application.
Typical X-ray tubes are bulky and fragile, and must be energized by heavy, high-voltage power supplies that restrict mobility. Thus, samples must be collected and brought to the X-ray unit for analysis. This is very inconvenient for popular X-ray applications. Certain xe2x80x9cfield applicationsxe2x80x9d include X-ray fluorescence (XRF) of soil, water, metals, ores, well bores, etc., as well as diffraction and plating thickness measurements.
A popular approach to portability of low power X-ray sources is the use of 109Cd which emits silver K X-ray lines during radioactive decomposition. There are many such instrumental sources currently in use, and software has been developed to make XRF with the silver line sensitive and reliable. Unfortunately the intensity of emission from 109Cd decays exponentially with a half-life of about 1.2 years. This necessitates frequent recalibration and eventual disposal of the 109Cd. The size of such radioactive sources are 1 or 2 Curies, so a license is required for transportation and possession of this isotope in the quantities useful for XRF.
Miniature size X-ray tubes have been demonstrated for medical purposes. For example, see U.S. Pat. Nos. 5,729,583 and 6,134,300. The geometry, however, is wrong for analysis. Such tubes are designed to send X rays into at least xcfx80 steradians for therapeutic reasons, rather than concentrating radiation in a spot that is easily accessed by a detector. Thus, such therapeutic X-ray tubes are inadequate for XRF work in the field because of the divergent beams. Another type of medical tube is a combination device where the X rays are for diagnostic purposes, i.e. tube placed in the body internally. Emitted X rays pass through tissue to film that is external to the body. This reveals the position of tumors or anatomic maladies. For example, see U.S. Pat. Nos. 5,010,562 and 5,117,829. With respect to the ""562 patent, it is important to note that the foil is not a transmission type anode, but an electron window. With respect to the ""829 patent, an interesting nozzle is shown, but the rest of the apparatus is large and inadequate for mobile field work.
Another type of x-ray tube includes a rod anode used for insertion into pipes and boilers for X-ray inspection. The metal anode rod is hollow from the point the electron beam enters to its opposite end, which is the target for the production of X rays. The whole rod structure is at the anode potential. A window in the side of the rod allows X rays to be emitted from the anode. To focus the electron beam on the target at the end, a magnetic coil is positioned along the rod. This electromagnet is heavy and requires considerable power from a large battery if it is to be portable. What is more, a long anode is of little value in typical analytical applications. Such rod-anode tubes are not of the transmission type.
To obtain a source of X rays that is of small diameter at the anode target of an X-ray tube, electrodes or apertures or both have been used in the tube. These are designed to focus the electron beam to a small spot on the target. One of these electrodes is termed a Wehnelt aperture. It is near the cathode, as is done in electron guns for microscopes. This seriously limits the electron flux. It is more important to limit the diameter of the electron beam where it strikes the anode, since this is the proximal source of X rays intended to strike a small portion of the analyte. This typically requires other electrodes. One type is a focusing electrode extending from the cathode region to approximately half way to the anode. This typically cylindrical tube reduces the distance between points of high and low voltage and it can lead to electrical breakdown in the tube.
An important feature of an X-ray tube used to excite X-ray fluorescence for elemental analysis is that the point where the X rays are generated be as close as possible to the sample being irradiated. This is necessary because the intensity of the X rays drops off in proportion to the reciprocal of the square of the distance from the target spot. It is a further advantage if the X-ray flux is focused to a small spot on the sample for reasons of spatial resolution, which allows analysis of discrete, small portions of a complex sample. In XRF, this X-ray beam is used to excite elements in the sample. They, in turn, fluoresce characteristic radiation in a Lambertian pattern, so XRF sensitivity is maximized if there is an angle of about 45xc2x0 between the beam illuminating the analyte and the fluoresced X rays going to the detector. For generic X-ray tubes, the spot impacted by electrons is broad and blunt, so the detector must be placed to one side with an angle that is 90xc2x0 or more instead of the desired 45xc2x0.
An object of the Treseder patent (U.S. Pat. No. 6,075,839) is to make the target accessible to the sample, but the exit window end of this invention is necessarily broadened (greater than 20 mm). In addition, the anode is seriously recessed from the window because the tube""s electron gun is placed at the side of the anode instead of generally behind it. What is more, it is impossible to modify the Treseder design because the target must be well separated from the X-ray window to make room for the curvature of the electron beam. The result is a large distance between the target and the sample, as shown in FIG. 3 of that patent.
Another requirement for sensitive XRF is irradiation with the correct band of wavelengths for exciting the sample. Higher bias voltage not only increases X-ray flux, but it changes the spectrum of the output. The bias should be subject to selection by the operator, and this setting should be independent of the tube current setting. In general, the higher the X-ray flux (and corresponding tube current), the more sensitive and accurate will be the measurements, whether they are XRF, plating thickness, or diffraction. However, once the detector is saturated, additional power is of no use. The current of the electron beam should be adjusted to produce adequate but not excessive x-ray intensity.
For generic X-ray tubes, substantial cooling is required because upon electron impact, and less than 1% of the electron beam power is converted to X-ray power. The rest of the energy becomes heat in the target. Heat also arises from thermionic electron sources. The heat cannot be allowed to accumulate and raise the temperature of the tube because high temperature decreases the lifetime of several tube parts. Thermal shock is especially destructive. Therefore, when operating at sufficient power, most X-ray tubes need to be cooled with a flowing liquid or forced air. The cooling effectiveness is limited primarily by the slow conduction of thermal energy through thick portions of the tube (e.g. the anode, in particular). Miniaturization reduces this problem to some extent, but cooling is still required for the inventions of U.S. Pat. No. 6,075,839 (cooling by oil, SF6, or forced air) and U.S. Pat. No. 6,044,130 which has exterior protrusions to aid in cooling by forced air. To obtain sufficient X-ray flux, all of the currently available X-ray tubes must be so large that they must be cooled. That is, a sufficiently powerful tube that is cooled only by ambient air is currently unavailable.
Another important feature is stability of the X-ray flux over the period of time required to calibrate the tube and measure the samples. This stability should be of the order of xc2x10.1%. Typical small high-voltage DC power supplies do not meet this criterion, and the resistivity of the tube can change over short periods of time. Thus, high-voltage stability presents a problem for mobile X-ray tubes.
Although X-ray tubes were first constructed over 100 years ago, no mobile X-ray tube is available for mobile applications such as those addressed by 109Cd radioactive sources. This is surprising because so many types of X-ray instruments are in use in science and industry. There is clearly a long-felt need for mobile, electronic, X-ray tubes and instrumentation.
It has been recognized that it would be advantageous to develop a mobile, miniature x-ray source. In addition, it has been recognized that it would be advantageous to develop an x-ray source for field applications. In addition, it has been recognized that it would be advantageous to develop a low-power consumption x-ray source. In addition, it has been recognized that it would be advantageous to develop an x-ray source that is not radioactive. It has also been recognized that it would be advantageous to develop an x-ray source with improved life or durability.
The invention provides a mobile, miniature x-ray source with a low-power consumption cathode element for mobility, and/or an anode optic creating a field free region to prolong the life of the cathode element. The x-ray source includes an evacuated tube. An anode is disposed in the tube and includes a material to produce x-rays in response to impact of electrons. A cathode is disposed in the tube opposing the anode. An electric field is applied to the anode and cathode. The cathode includes a cathode element to produce electrons that are accelerated towards the anode in response to the electric field between the anode and the cathode. A power source is electrically coupled to the anode, the cathode, and the cathode element. The power source provides power for the cathode element, and provides the electric field between the anode and the cathode.
In accordance with a more detailed aspect of the present invention, the tube is configured to be both miniature and mobile. The tube can have a length less than approximately 3 inches, and a diameter or width less than approximately 1 inch. The cathode element can include a low-power consumption cathode element with a low power consumption less than approximately 1 watt. The power source can include a battery power source.
In accordance with another more detailed aspect of the present invention, the battery power source provides an electric field between the anode and the cathode of at least approximately 15 kilo-volts.
In accordance with another more detailed aspect of the present invention, a window can be disposed in the evacuated tube at the anode. The window can be aligned with a longitudinal axis of the evacuated tube to release x-rays substantially along the longitudinal axis. Alternatively, the window can be disposed in a side of the evacuated tube to release x-rays transverse to the longitudinal axis.
In accordance with another more detailed aspect of the present invention, a field-free region can be positioned at the anode to resist positive ion acceleration back towards the cathode element. The electrons can impact the anode and heat the anode, releasing residual gas molecules. In addition, the electrons can ionize the residual gas molecules to positive ions. Such ions would normally be accelerated back to the cathode and sputter-erode the cathode element.
In accordance with another more detailed aspect of the present invention, an anode tube can be disposed at the anode between the anode and the cathode, and electrically coupled to the anode so that the anode and the anode tube have the same electrical potential. The anode tube can create the field-free region.
In accordance with another more detailed aspect of the present invention, a cathode optic can be disposed proximate the cathode element. The cathode optic can including a plate with an aperture therein configured to allow electrons to pass through the aperture towards the anode.
Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention.